1. Field of the Invention
The present invention relates to an image forming apparatus, an image forming method, and a sheet feeding apparatus, and more particularly to an image forming apparatus, an image forming method, and a sheet feeding apparatus capable of effectively feeding sheets.
2. Description of the Background Art
A background image forming apparatus, such as a copying machine, a printer, or a facsimile machine, generally includes a sheet feeding apparatus for feeding sheets one by one from a paper tray. The sheet feeding apparatus generally includes a mechanism for separating an uppermost sheet from the other sheets fed from the paper tray and feeding only the uppermost sheet when a plurality of sheets are fed from the paper tray. As one of the mechanism, a friction reverse roller method is known. The sheet feeding apparatus using the friction reverse roller method includes a feeding roller and a separating roller as the mechanism for separating the uppermost sheet from the other sheets fed from the paper tray and feeding only the uppermost sheet.
FIG. 1 illustrates a sheet feeding apparatus 20b which is one example of the sheet feeding apparatus using the friction reverse roller method. The sheet feeding apparatus 20b includes a bottom plate 9b, a pick-up roller 4b, a feeding roller 2b, a separating roller 3b, a grip roller 5b, and a sensor 6b. 
The bottom plate 9b is disposed in a paper tray 1b of the image forming apparatus and loads sheets S. The pick-up roller 4b feeds the sheets S loaded on the bottom plate 9b from the paper tray 1b. The feeding roller 2b rotates in a sheet conveyance direction. The separating roller 3b faces and pressingly contacts the feeding roller 2b and is driven to rotate in a direction opposite the sheet conveyance direction. The bottom plate 9b moves upward in a direction U to cause an uppermost sheet S of the sheets S loaded on the bottom plate 9b to contact the pick-up roller 4b. 
The pick-up roller 4b is connected to the feeding roller 2b via gears and rotates to feed the uppermost sheet S toward a nip formed between the feeding roller 2b and the separating roller 3b. The feeding roller 2b further feeds the sheet S toward the grip roller 5b. When the sheet S reaches the grip roller 5b, the pick-up roller 4b moves away from the sheet S. The feeding roller 2b stops rotating before a tail portion of the sheet S passes the nip formed between the feeding roller 2b and the separating roller 3b to prevent the rotating feeding roller 2b from successively feeding a next uppermost sheet S of the sheets S loaded on the bottom plate 9b and thereby interfering with proper image forming operations.
The feeding roller 2b includes a roller, a shaft, and a one-way clutch. The clutch switches on and off connection between the roller and the shaft. When the clutch switches off the connection, the shaft does not rotate the roller because a driving force of the shaft is not transmitted to the roller. Therefore, the feeding roller 2b is driven to rotate in the sheet conveyance direction by the sheet S fed by the grip roller 5b without disturbing proper conveyance of the sheet S.
The sensor 6b is disposed downstream of the grip roller 5b in the sheet conveyance direction. When the sensor 6b detects a head of the previous sheet S, feeding of the next uppermost sheet S starts so that the next sheet S is stably conveyed. The pick-up roller 4b and the feeding roller 2b are driven at a predetermined timing when the next sheet P is fed to enable image forming operations at a predetermined print speed without contacting the tail of the previous sheet S.
The background image forming apparatus tends to suppress the speed at which the sheet S is fed during image forming so as to form a high-resolution image. However, the background image forming apparatus is still required to provide a higher print speed. To respond to such requirement, the distance between the sheet S and the next sheet S is reduced in the background image forming apparatus.